The market for devices having touch-sensitive screens is increasing very rapidly. Many of these devices are mobile devices with touch screens, such as, mobile communications devices. Other such devices include multi-touch displays (that is, display devices which are able to detect the respective positions on the screen of three or more distinct contacts) and computers with touch-sensitive screens.
Touch sensitive screens use charge coupled device image sensors. The sensors often employ a double ITO structure. Such a structure has a first transparent, conductive ITO electrode separated from a second transparent, conductive ITO electrode by a thin insulating layer of silicon dioxide or silicon dioxide doped with a conductive metal. The double ITO structure is produced by forming sequential layers of ITO, SiO2, and ITO on a transparent or semi-transparent substrate, and then selectively etching the layers. The remaining portions of the double ITO structure provide electrodes on the surface of the substrate.
The steps of etching the different layers are a crucial step in fabricating a microelectronic component. One method widely employed for etching an article carrying a layer to be etched is to overlay the layer with a suitable patterned mask and then to immerse the layer and mask in a chemical solution that attacks the layer while leaving the mask intact and while only etching other materials of the article to a minimal extent. This process is conventionally applied to the etching of a double ITO structure comprising sequential layers of ITO, SiO2, and ITO, by performing multiple etching steps for the different respective layers.
Specifically, the method used to etch each ITO layer has been by immersion in a hot acidic solution, such as, hydroidic or nitric acid solutions, or a hydrochloric acid solution with a concentration such as 2N. Such an acid etches the material isotropically and may strip away the photo-resist, potentially leading to etching of incorrect portions of the layers beneath. US 20080217576 (Stockum et al) discloses the use of phosphoric acid based etching paste to etch oxidic conductive layers.
U.S. Pat. No. 5,456,795 (Danjo et al) discloses using an aqueous mixture of etchant containing hydroiodic acid and ferric chloride for etching ITO to form a minute electrode pattern.
Most etchant compositions for ITO acidic cannot etch silicon dioxide. Instead, a silicon dioxide layer is conventionally etched using an etching composition which contains environmentally unfriendly fluorine-containing compounds, such as, HF or NH4F. The mechanism for the etching of the silicon dioxide is dissolution of the silicon dioxide. For example, U.S. Pat. No. 5,976,988 (Konuma et al) describes a fluoride based etching solution to etch silicon oxide, alumina and a silicon nitride film. U.S. Pat. No. 6,254,796 (Rath et al) describes selective etching of silicate glass using a fluoride-containing compound and certain organic solvents. U.S. Pat. No. 6,936,183 (Chinn et al) describes isotropically etching a silicon oxide layer sandwiched between two silicon-containing layers with a gaseous hydrogen fluoride-water mixture. U.S. Pat. No. 7,470,628 (Ko) describes the use of fluorocarbon gases for etching silicon dioxide.
Etching an SiO2 layer on an article is a particularly difficult if the other portions of the article also include SiO2 or its alloys, since the other portions of the article are also exposed to the etching composition.
Note that these referenced documents are concerned with etching either ITO or SiO2, not both.
FIGS. 1A and 1B illustrate two problems which may occur when a double ITO structure is etched using a patterned photo-resist mask. Ideally, the unetched portion of the double ITO structure (that is, the portion of the double ITO structure which remains after the etching) should have a pattern very similar to that of the patterned photo-resist mask. However, if the double ITO structure is etched in locations below the side portions of the photo-resist mask, the result is called “over etching”. This is illustrated in FIG. 1A where the unetched portion 22 of the double ITO structure is narrower than the photo-resist mask 21 in the lateral direction (that is, in the direction parallel to the surface of the substrate 23). Over-etching leads to lead lifting (i.e. separation of the electrode from the substrate) as the leads become too thin. Conversely, FIG. 1B illustrates a problem called “under-etching”, in which the unetched portion 22 of the double ITO structure extends laterally beyond the photo-resist mask 21. Under-etching results in short circuits since adjacent leads are shorted.